Such ram boring machines are mainly used to lay water supply pipes or electricity or telephone cables without having to tear up the road surface or the sidewalk and dig a trench. They usually have an impact tip that is struck by a pneumatically operated striker piston. To drive the impact tip forwards the striker piston is moved to and fro in a tubular housing by compressed air, supplied through a compressed air line, acting alternately on the front and rear faces of the striker piston.
Most ram boring machines driven by compressed air are turned on and off by means of ordinary stopcocks, e.g. ball-valves.
Since relatively long hoses, from about 20 m to 100 m long, extend from the stop-cock at the source of compressed air to the ram boring machine, the pressure in the hose and in the ram boring machine builds up relatively slowly. As a result the striker piston can be moved unfavourably so that the front and back faces of the striker piston are subjected to the same pressure, resulting in a dead centre position from which the striker piston can only be caused to start up by a jolt with compressed air. This can be brought about for example by kinking the compressed air hose nd abruptly bending it back. However, this measure is ineffective if the ram boring machine has already penetrated so far into the ground that the length of the hose in the bore hole is more than 15 m to 20 m. Great problems are thus caused in the case of fairly long bore holes by starting difficulties.